1. Field of the Invention
The present invention relates to a bandwidth allocation method, an optical line terminator, a subscriber station, a communication system, and a recording medium recording a program of a device in a system, such as Passive Optical Network (hereinafter “PON”), in which a plurality of devices are connected to the optical line terminator.
2. Description of Related Art
In recent years, the broadband of access lines has progressed as a result of the rapid spread of the Internet. Examples of existing broadband access lines include various systems such as ADSL and cable modems. The PON is promising worldwide for wider bandwidth.
Meanwhile, the Worldwide Interoperability for Microwave Access (hereinafter “WiMAX”) is attracting attention as a broadband wireless communication system in regions where the installation of metal lines or optical fibers is difficult. Recently, the Fixed Mobile Convergence (FMC) has been proposed to simplify the system and reduce the cost, and a network for connecting the WiMAX to the subscribers of PON has been proposed (see Non-Patent Document 1 (Gangxiang Shen et. al., “Fixed Mobile Convergence Architectures for Broadband Access: Integration of EPON and WiMAX”, IEEE Communications Magazine August 2007, pp. 44-50)).
FIG. 1 shows a general configuration of the PON. ONUs (optical network unit) are installed in end users' houses, and an OLT (optical line terminator) is installed at a station. The ONUs and OLT are connected by optical fibers and an optical splitter. The personal computers of the users are connected to the network through the ONUs and further connected to superior networks and the Internet through the OLT.
As uplink signals (wavelength is usually 1.3 μm) and downlink signals (wavelength is usually 1.5 μm) are wavelength-multiplexed, the devices are connected by interactive single-core optical fibers. The downlink signals are broadcasted from the OLT to all ONUs, and each ONU checks the address of the frame and imports the frame addressed to the ONU.
The uplink signals from the ONUs merge at the optical splitter, and time-division multiplexing is used to avoid collision of the signals. Therefore, the OLT adjusts output requests (REPORT) momentarily reported from the ONUs and provides signal transmission permissions (GATE) to the ONUs after calculating the transmission time based on the distances between the OLT and the ONUs.
The output request (REPORT) includes information of queue status (length of queue) of buffers. The signal transmission permission (GATE) includes transmission start time and transmission duration time for each priority of signals, and the ONUs transmit the uplink signals according to the times. Thus, the uplink bandwidth allocation is realized by allocation of time slots. FIGS. 2 and 3 show flows of signals in which three ONUs are connected. FIG. 2 shows downlink signals, while FIG. 3 shows uplink signals. Rectangles with numbers denote frames of ONU-addressed signals and ONU-departed signals.
FIG. 4 shows a time relationship of an output request signal (REPORT), an output enable signal (GATE), and an uplink data signal (DATA) exchanged between the ONU and the OLT. FIG. 4 shows a signal exchanged between one ONU and one OLT. In FIG. 4, t1 and t5 denote transmission time of REPORT, t2 and t4 denote arrival time of GATE, Waiting Time denotes waiting time until signal transmission, and Time Slot denotes a time slot of data transmission. In many cases, REPORT is transmitted at the end of DATA by piggy back. In that case, t4=t5.
FIG. 5 shows a time relationship of signals exchanged between three ONUs (ONU1, ONU2, and ONU3) and the OLT. A cycle in which uplink signal transmissions of all ONUs are performed will be called a Service Cycle. The length of the service cycle is usually not constant and is often dynamically changed according to the output requests from the ONUs.
Ethernet (registered trademark) and PON are standardized by IEEE802.3ah, wherein frame formats of a REPORT message and a GATE message are defined. However, uplink bandwidth allocation methods or algorithms are not defined and are left up to the installation of devices.
FIG. 6 shows a system configuration of WiMAX. BS and SS are respectively called a Base Station (or master station) and a Subscriber Station (or substation). The former is installed at the service provider, and the latter is installed at the user's house. The base station device BS and the subscriber station SS are wirelessly connected, and services such as connecting the Internet are provided to the subscriber.
The specifications for WiMAX are defined by IEEE802.16 series. Although there are various kinds of frequency bands and modulation methods in the physical layer, the MAC layer is shared. The uplink signals and the downlink signals are switched in a time-division manner, and a plurality of SS signals (uplink and downlink) are also multiplexed in a time-division manner.
As in the PON, the downlink signals are broadcasted from the BS to all SSs, and each SS checks the address of the frame and imports the frame addressed to the SS. The uplink signals are also basically the same as in the PON, and the BS adjusts bandwidth allocation requests from the SSs and returns allocation results to the SSs.
In general, an allocation module (hereinafter “AM” in Description and Figs.) mounted on the OLT intensively allocates the uplink bandwidth of PON based on the requests from the ONUs. An allocation module (AM) mounted on the BS also intensively allocates the uplink bandwidth of WiMAX based on the requests from the SSs.
The biggest difference between the systems of Ethernet PON (hereinafter “EPON”) and WiMAX is that the former is a connectionless communication system, while the latter is a connection communication system. Thus, in relation to the bandwidth allocation requests, EPON is queue class based, while WiMAX is connection based.
FIG. 7 shows a configuration of a network in which EPON and WiMAX are integrated. WiMAX is arranged under the ONUs of the PON. The ONU and the BS are integrated as an optical network base unit. The optical network base unit will be called an ONU-BS.
An example of a related art by the present applicant includes a technique in which an access point is wirelessly connected to a mobile terminal, an access line connecting device allows the mobile terminal to access the Internet, and packets are routed, the routing unlinked with the access to the Internet by the subscriber (see, for example, Japanese Patent Laid Open Publication No. 2005-64783).
Problems of the conventional techniques will now be described.
In the network in which WiMAX and EPON are connected, the ONU-BS needs to convert an uplink bandwidth allocation request of WiMAX to a request of EPON.
As described, the compatibility between the queue class base of the EPON and the connection base is low. Therefore, efficient and accurate reflection of the requests of distal SSs in the bandwidth allocation by the OLT is difficult.
Furthermore, as the numbers of the SSs and the ONUs increase, the load of the bandwidth allocation processing by the BSs and the OLT increases, resulting in a problem of scalability. More specifically, the allocation requests are gathered from all SSs or ONUs in the conventional allocation system, and the control circuit then intensively performs allocation until the next service cycle starts. Therefore, an excessive load is imposed on the control circuit when a large number of SSs or ONUs exist. Thus, expensive and fast integrated circuits or CPUs are required for the control circuit in large-scale WiMAX or PON, which may lead to an increased cost of the system. Furthermore, if a plenty of calculation time is allowed, the start of the service cycle is delayed and the bandwidth is wasted, causing degradation of performance.
For efficient and highly accurate reflection of the requests of distal SSs, there is a method, as shown in FIG. 8, of transmitting the requests of the SSs to the OLT as it is, and the OLT intensively performs allocation. However, with the configuration, there is a problem that the load of processing by the OLT is significantly increased.
Furthermore, the technique in Japanese Patent Laid Open Publication No. 2005-64783 attempts to provide a public Internet connection service to a wide area. The technique is not designed not to require expensive and fast integrated circuits or CPUs in the control circuit even in a large-scale system with a large number of terminal devices.